The present invention pertains generally to plasma enhanced bonding of metallic materials, and more particularly, to a method and apparatus for using hot plasma gas to simultaneously heat, clean, and activate the bonding surfaces before and during bonding of a circuit package to a circuit board.
Various techniques exist for bonding metals together, including welding, brazing, soldering, and more recently, direct contact and diffusion bonding.
Welding is a metal joining process wherein coalescence is produced by heating the metal to suitable temperatures, with or without the application of pressure and with or without the use of filler metals. Coalescence is a growing together into one body. A weld is the junction of members or the edges of members which are to be joined or have been joined by melting and refreezing at the contact interfaces of members themselves. Filler metal is the material to be added in making a welded, brazed, or soldered joint. Base metal is the material to be welded, soldered, or cut.
The term arc welding applies to a large and varied group of processes that use an electric arc as the source of heat to melt and join metals together as the molten metal re-freezes. In arc welding processes, the joining of metals, or weld, is produced by the extreme heat of an electric arc drawn between an electrode and the work-piece, or between two electrodes. The formation of a joint between metals being arc welded may or may not require the use of pressure or filler metal. The arc is struck between the workpiece and an electrode that is mechanically or manually moved along the joint, or that remains stationary while the workpiece is moved underneath it. The electrode is either a consumable wire rod or a nonconsumable carbon or tungsten rod which carries the current and sustains the electric arc between its tip and the workpiece. When a nonconsumable electrode is used, a separate rod or wire can supply filler material, if needed. A consumable electrode is specially prepared so that it not only conducts the current and sustains the arc, but also melts and supplies filler metal to the joint, and may produce a slag covering as well.
Gas welding processes are a group of welding processes in which a weld is made by heating with a gas flame. Pressure and/or filler metal may or may not be used. Also referred to as oxyfuel gas welding, the term gas welding is used to describe any welding process that uses a fuel gas combined with oxygen, or in rare cases, with air (20% Oxygen), to produce a flame having sufficient energy to melt the base metal. The fuel gas and oxygen are mixed in the proper proportions in a chamber, which is generally a part of the welding tip assembly. The torch is designed to give the welder complete control of the welding flare, allowing the welder to regulate the melting of the base metal and the filler metal. The molten metal from the work-piece edges and the filler metal intermix in a common molten pool and join upon cooling to form one continuous piece.
Brazing and soldering are welding processes in which materials are joined by heating to a suitable temperature and by using a filler metal with a melting point below that of the base metal. The filler metal is distributed to the closely fitted surfaces of the joint by capillary action.
Resistance welding consists of a group of processes in which the heat for welding is generated by the resistance to the electrical current flow through the parts being joined, using pressure. It is commonly used to weld two overlapping sheets or plates which may have different thicknesses. A pair of low resistance electrodes conducts electrical current through the sheets, forming a weld. The Key aspect of the resistance weld is that most of the resistance in the welding circuit is in the contact resistance where the metals are pressed together, so that most of the (1{circumflex over ( )}2 *R) heat is formed at the surfaces that are to be welded together.
The properties of a welded joint depend partly on the correct preparation of the edges being welded. Cleanliness is of key concern. All mill scale, rust, oxides, and other impurities must be removed from the joint edges or surfaces to prevent their inclusion in the weld metal. The edges should be prepared to permit fusion without excessive melting. Care must be taken to keep heat loss due to radiation into the base metal from the weld to a minimum. A properly prepared joint will keep both expansion on heating and contraction on cooling to a minimum.
Diffusion bonding is a method of joining metallic or non-metallic materials. This bonding technique is based on the atomic diffusion of elements at the joining interface. Diffusion process actually is the transport of mass in form of atom movement or diffusion through the lattice of a crystalline solid. Diffusion of atoms proceeds by many mechanisms, such as exchange of places between adjacent atoms, motion of interstitial atoms or motion of vacancies in a crystalline lattice structure. The latest is the preferable mechanism due to low activation energy required for atom movement. Vacancy is referred to an unoccupied site in a lattice structure. Both Diffusion and Direct contact bonding are the preferable bonding methods. Direct Contact/Fusion bonding occurs when the surface atoms are brought within atomic distances with the application of pressure and heat. Any surface contamination will inhibit bonding.
Diffusion bonding involves diffusion of atoms via a thermodynamic process where temperature and diffusibility of the material are considerable parameters. In general, the diffusion rate, in term of diffusion coefficient D, is defined as D=Doxe2x88x92Q/RT, where Do is the frequency factor depending on the type of lattice and the oscillation frequency of the diffusing atom. Q is the activation energy, R is the gas constant and T is the temperature in Kelvin.
The activation energy for atomic diffusion at the surface, interface and grain boundaries is relatively low compared to the bulk diffusion due to a looser bond of the atoms and higher oscillation frequency of the diffusing atom. This enhances the atomic diffusion, and thus eases the diffusion bonding of two metal pieces assuming that a perfect interface contact exists.
Since diffusion bonding is driven by the diffusion of atoms, diffusion bonding process can be used to bond dissimilar materials that are difficult to weld, such as, steel and copper alloys. When bonding metals together, direct contact/fusion and diffusion bonding causes micro-deformation of surface features, due to the pressure and temperature applied, which leads to sufficient contact on an atomic scale to cause the materials to bond. However, before the materials can be joined they undergo an extensive preparatory treatment.
The interface contact can be optimized by a treatment of the surface to be bonded through a number of processes, such as mechanical machining and polishing, etching, cleaning, coating, and material creeping under high temperature and loading.
All of the above-mentioned bonding methods are problematic due to the inability to prevent contaminants from adhering to the bond site surfaces. Surface contamination causes poor bond adhesion, resulting in less robust bonds and therefore less reliable interfaces. Accordingly, much effort has gone into cleansing the surfaces of metals prior to being bonded.
Low temperature plasmas of various ionized gases can be used to reactively etch/ash organic materials found on the surface of materials. In this procedure, typically the material is placed in an RF cavity with a flowing reactive gas. The nature of the gas selected is chosen based upon the desired effect. Oxygen or argon is generally used, however, specific gases (CF4) may be used to tailor the reaction for the desired effect. By removing surface contamination, plasma cleaning increases the bonding or adhesive properties of the bondsite surface.
A plasma is a collection of positive, negative, and neutral particles wherein typically the density of the negatively charged particles is equal to the density of the positively charged particles. When an energetic electron strikes a neutral gas molecule, it can cause dissociation and form free radicals and ions. The free radicals cause chemical reactions for destroying contaminants. For example, with oxygen, the dissociation process produces the free radical atomic oxygen (O). This reactive species has enough energy to break a carbon-carbon bond. When the plasma gas is a mixture of hydrogen and argon, for example, the free radical atomic oxygen (O) can therefore combine with the hydrogen (H2) into water (H2O).
Prior art plasma cleaning involves placing the workpieces to be bonded into a plasma chamber, creating a vacuum within the chamber by pumping out all the air from the chamber, introducing a gas or gaseous mixture into the chamber, and energizing the gas in the chamber to produce the plasma. In the presence of the plasma, organic contaminants on the bondsite surfaces are converted to carbon monoxide, carbon dioxide, water vapor, and/or other gasses, which are pulled out of the plasma chamber by a vacuum pump. After a predetermined amount of time, the gas flow and energy are shut off, and the chamber is then purged with a nonreactive gas, such as nitrogen, to remove all traces of volatile compounds. Finally, the chamber is returned to atmospheric pressure. A cleaning cycle usually lasts from between 30 seconds to 15 minutes or more and is largely a function of the workpiece loading in the plasma chamber.
The current plasma cleaning methodology is problematic. First, immediately upon emerging from the plasma cleaning chamber, the cleaned workpiece(s) are re-exposed to surface contamination due to the organic particles in the air. In addition, the separate plasma cleaning chamber and equipment, coupled with the significant amount of time required to set up and execute the cleaning process, is quite costly.
It is therefore an object of the invention to provide a method for simultaneously heating, cleaning, and bonding materials together.
The present invention is a novel plasma enhanced packaging method and apparatus. The invention effectively generates a plasma bath, or xe2x80x9cbubblexe2x80x9d, around the metal bonding surfaces of the package lid and circuit board prior to and during the bonding action to thereby simultaneously heat, clean, and activate the surfaces of the materials that are to be bonded together, while the bonding is taking place.
In accordance with the invention, a package lid is sealed over electronic circuitry to protect and preferably shield the circuitry enclosed therein. This lid provides, mechanical, electrical, and environmental shielding to the circuitry within. In this embodiment, the circuit board or package bottom containing the circuitry to be enclosed is positioned securely on a platen mounted on a press base. The exposed peripheral bonding surface of the circuit board/package bottom surrounding, or containing the circuitry to be enclosed that will contact and be bonded to the bonding edges of the metal lid is either a metal laminate, package bottom or printed metal layer. The lid is positioned over the circuitry such that its bonding edges are aligned with the aforementioned peripheral bonding surface. Prior to bonding the lid to the package base/circuit board, hot plasma is applied in a pressurized directional modulated flow between the lid bonding edges and the metal periphery bonding surface around the circuitry to be enclosed. While continuing to apply the modulated plasma flow, a press applies vertical downward pressure on the edges of the metal lid to diffusion bond/direct contact bond the lid edges to the metal periphery bonding surface surrounding the circuitry to be enclosed. The plasma flow and pressure are then removed, resulting in enclosed circuitry sealed within under the lid.
Preferably, the plasma cleans and activates the surface while heating the bonding surfaces to allow direct contact bonding to take place as the clean metal surfaces are pressed together sufficiently such that at least some of the surface atoms are brought within atomic distances, and without melting the bonding surfaces. This eliminates the need for an additional heating mechanism for heating the bonding surfaces, thereby preventing component de-soldering or damage to the surrounding circuitry from overheating.
Accordingly, the present invention advantageously provides a novel technique that cleans the metal surfaces to be bonded as they are bonded to eliminate any contaminants from being introduced into the final bond, thereby improving the bond adhesion properties. The invention not only allows differing metals to be bonded together, but may also allow materials to be bonded together that prior to the present invention were difficult to bond due to contaminant compounds formed at the bond site, which resulted in weaker bonds. In addition to the above-named advantages, the temperature of the plasma/hot gas is high enough that the press need not include a heater. The invention essentially provides xe2x80x9clocalizedxe2x80x9d heating directed only at the bonding surfaces, thereby reducing the risk of damage to the circuitry to be enclosed as well as any surrounding circuitry. The invention also makes possible the bonding of non-metallic materials such as sapphire, silicon, ceramic, and quartz materials in a similar manner. The direct contact bonding method only requires that the surface finish, temperature, and pressure are adequate for enough of the contact surface to be brought close enough together for the atoms from one surface to bond the atoms in the opposite surface while a modulated stream of intense plasma cleans, heats, and activates those surfaces as the contact happens.